Upon exposure to lysates or supernatants of necrotic transformed cell lines, human dendritic cells (DCs) undergo maturation. In contrast, DCs exposed to apoptotic transformed cell lines or necrotic lysates of primary cells remain immature. Analysis of supernatants of necrotic transformed cell lines showed them to be enriched in the heat shock proteins (hsp) 70 and gp96, in contrast to supernatants of primary cells. Likewise, cells from a variety of primary human tumors contained considerably higher levels of hsp than their normal autologous tissue counterparts. Of the majority of human tumors enriched in hsps (hsp70 and/or gp96), their corresponding lysates matured DCs. The maturation effect of tumor cell lysates was abrogated by treatment with boiling, proteinase K, and geldanamycin, an inhibitor of hsps, suggesting that hsps rather than endotoxin or DNA were the responsible factors. Supporting this idea, highly purified, endotoxin-depleted hsp70, induced DC maturation similar to that seen with standard maturation stimuli LPS and monocyte conditioned medium. These results suggest that the maturation activity inherent within tumor cells and lines is mediated at least in part by hsps. The release of hsps in vivo as a result of cell injury should promote immunity through the maturation of resident DCs.